1. Field of the Invention
Exemplary embodiments of the present disclosure relate to a HART analog input module with a differential input stage, and more particularly to a HART analog input module with a differential input stage configured to efficiently perform a HART communication while maintaining a differential structure improved in noise characteristics relative to an analog signal.
2. Description of Related Art
An analog input module is widely used at industrial sites including food packing machines, various industrial furnaces, semiconductor manufacturing devices and plastic forming machines, and concomitant with increased demands on high performance, intelligent transmitters and effective management thereon, a communication function between the transmitter and PLCs (Programmable Logic Controllers) is required of late.
The PLC is a general-purpose control device configured to enable a program control by replacing the relay, timer and counter functions of the conventional control panel with a semiconductor device including integrated circuits and transistors, and by adding an operation function to a basic sequential control function.
A current communication method widely adopted by a transmitter complies with HART (Highway Addressable Remote Transducer) communication protocol. Generally speaking, the HART communication protocol supports a combined digital and analog signal on a dedicated wire or set of wires, in which on-line process signals (such as control signals, sensor measurements, etc.) are provided as an analog current signal (e.g., ranging from 4 mA˜20 mA), and has an excellent improvement effect in information use and management of 2-wire type instruments.
The HART communication protocol supports an output of a measurement value (process variable) measured in an analog signal and added information to a digital signal. The HART communication protocol supports setting-up of various parameters, calibrates a transmitter at a remote area, and even performs fault diagnosis, whereby a field situation can be three-dimensionally monitored from a control room with ample information related to operation of field instruments.
Use of the rapidly developed PLC has gradually expanded into an area of distributed control system of late, and as a result, the HART communication function largely used in the distributed control system is also required in the PLC.
The HART protocol makes use of the FSK (Frequency Shift Keying) standard to superimpose the digital signals at a low level on top of the 4 mA-20 mA analog signals. The HART protocol communicates at 1200 bps without interrupting the 4 mA˜20 mA signals and allows a host application (master) to get two or more digital updates per second from a field device. As the digital FSK signal is phase continuous, and an average value of the HART communication signal is zero, there is no interference with the 4-20 mA analog signals.
Meanwhile, in a case an analog input module used in industrial instrument such as the PLC is connected to various HART field instruments, the product must be equipped with a function of converting an analog signal to a digital signal and a function of performing the HART communication with HART field instruments as well.
Creation and interpretation of a HART communication signal by an analog input module is carried out through a HART modem. In order to normally implement the function, a ground potential of the HART communication signal must be continuously maintained until transmitted from an input terminal of the analog input module to the HART modem.
Although a single-ended type input terminal provides a structure required for this condition, a problem of disrupting an insulation state among channels and a problem of abandoning a differential structure excellent in noise characteristic come to the surface. That is, in order to allow the HART modem to normally recognize a detected HART communication signal, potentials of the HART communication signal must be identical at each element processing the HART communication signal, and a single-ended configuration used for this purpose means that grounds of each HART field device connected to an analog input module are common.
For example, grounds of several HART field devices respectively connected to each channel are all commonly connected to a ground of the analog input module to allow operating on a same ground potential. This means that, in a case a defect occurs on any one of the grounds in the HART field devices, the defect may be transmitted to a ground of a structurally-different HART field device and to a ground of an analog input module.
Furthermore, as is known, a single-ended signal input circuit cannot remove a common mode noise, such that a path of a commonly-grounded circuit into which the common mode noise is introduced increases in proportion with the channels.
As one of the methods structurally solving the abovementioned problems, although each channel may be configured to have an individual signal processing circuit, the number of signal processing components is required as many as the number of channels to disadvantageously increase the manufacturing cost.